In the chlorination of titaniferous materials, as for example, titanium-containing ores such as ilmenite, by reacting chlorine and carbon with the ore at elevated temperatures to produce titanium tetrachloride, large amounts of dust are formed which comprise essentially particulate ferrous chloride contaminated with TiO.sub.2, coke and other metal oxides and chlorides, as for example, magnesium chloride and manganese chloride. The nature of this dust, hereinafter referred to as chlorinator dust, is such that it would constitute an environmental nuisance if stored or disposed of as land fill. It is desirable therefore to use the chlorinator dust as a source material for production of useful products such as iron oxide and gaseous chlorine, the latter, for example, for use in chlorinating titaniferous ores.
There are several known processes for recovering iron oxide and chlorine by oxidation of iron chlorides. For example, U.S. Pat. No. 3,325,252 discloses oxidizing a mixture of iron chlorides consisting essentially of ferric chloride in two stages, partial oxidation being carried out in the first stage by reacting the mixture of iron chlorides with oxygen at temperatures from 650.degree.-1000.degree. C. and then continuing the oxidation reaction in a second stage at temperatures above 450.degree. C. and preferably from 50.degree.-100.degree. C. below the temperature prevailing in the first stage of the process. However, this and similar processes for oxidizing ferric chloride are not regarded as pertinent to the process of the instant invention since the latter is directed specifically to the oxidation of chlorinator dust which is essentially particulate ferrous chloride contaminated with coke and other metal chlorides and oxides.
The prior art also discloses methods for oxidizing ferrous chloride to form iron oxide (Fe.sub.2 O.sub.3) and gaseous chloride. For example, in British Pat. No. 1,407,034 (German Pat No. 2,337,099) ferrous chloride in the vapor phase is reacted with oxygen in excess of that required stoichiometrically for conversion of the ferrous chloride to ferric oxide; and at temperatures sufficiently high to avoid condensation of the ferrous chloride; while U.S. Pat. No. 3,865,920 preheats ferrous chloride at 980.degree. to 1110.degree. C. and then contacts it with oxygen, thereby to form a mixture of iron chlorides, iron oxide, oxygen and chlorine - which mixture is cooled and the residual iron chloride converted to iron oxide and chlorine. U.S. Pat. No. 2,954,274 teaches oxidizing ferrous iron chloride by means of air or oxygen at temperatures from 400.degree.-1000.degree. C. in fluidized bed of iron chloride and optionally iron oxide (column 6, lines 3-10); and U.S. Pat. No. 3,793,444 describes oxidizing vaporous iron chloride by passing a mixture of the iron chloride and oxygen through several superposed zones subdivided by walls and in the presence of recycled inert solid particles. The process is thus concerned with oxidation of vaporous iron chloride and not chlorinator dust, that is to say, a mixture of particulate ferrous chloride plus coke and other metal chlorides and oxides; and requires relatively complicated equipment which gives rise to frequent operational difficulties.
With respect to the oxidation of chlorinator dust, many unexpected difficulties were encountered due to the physical characteristics of the dust. For example, efforts to oxidize the dust according to the vapor phase process of British Pat. No. 1,407,034 were unsuccessful due to the difficulty in volatilizing the dust. Thus when sufficiently high temperatures and energy were used to volatilize the dust the presence of the oxygen effected deposition of solid iron oxide on the walls of the reactor -- and these difficulties were increased due to the contaminants in the dust. For example, a typical chlorinator dust may comprise as high as 25% coke. Further, mechanical separation of the particulate ferrous chloride from the contaminants, prior to oxidation, is not feasible in as much as the ferrous chloride is firmly combined with the contaminants as a coating thereon. Moreover if, on the other hand, the dust is oxidized then the iron oxide so produced is highly contaminated and hence unsuitable for its intended use. And further, if oxidation of the dust is carried out at high temperatures, that is in excess of 800.degree. C., the coke present in the dust is burned up thereby producing hot spots in the reactor which effect sintering of the iron oxide accompanied by a buildup of the oxide on the walls which leads to clogging within a short time. Also, at these high temperatures the gaseous chlorine produced is strongly diluted with CO.sub.2 and hence is unsuitable for immediate recycling to the chlorinator and, obviously, efforts to concentrate the dilute chlorine involves great expense and hence are unacceptable. Moreover, it is difficult to apply required auxiliary energy from the outside of the reactor. A powerful heating-up of the walls of the reactor could easily lead to ignition of the coke which, in the presence of oxygen, would cause the oxidation reaction to proceed out of control. The aforementioned U.S. Pat. No. 3,325,252 teaches multistage oxidation of ferric chloride and while multistage oxidation of chlorinator dust at high temperatures was found to improve yield, it does not solve the problem of contaminating the end-products.
Finally, at the high temperatures taught by the prior art, the equilibrium for oxidation of the ferric chloride is far to the side of ferric chloride and hence complete oxidation is not effected. It has now been found however, that by conducting the oxidation of the chlorinator dust in successive stages and at relatively low temperatures, substantially complete oxidation of the iron chloride to ferric oxide will take place; and that the major portion of iron oxide and gaseous chlorine so formed will be in sufficiently pure form for commercial applications.